1. Field of the Invention
The invention relates to a photographing lens used in an electronic still camera or a digital video camera, and more particularly, to a bright telephoto lens system having an inner focus type.
2. Description of the Related Art
Recently, digital cameras or digital camcorders that use an image sensing device such as a charge coupled device (CCD) or a complementary metal-oxide semiconductor (CMOS) have been widely distributed.
In video cameras or digital cameras, a telephoto lens that is bright and has a small F number is preferred. However, a bright telephoto lens is big and heavy, and thus the focusing speed may be slow in a camera having an auto-focusing function.
To address the above problem, many focusing methods have been suggested; however, these may not provide a lens which is both bright and light weight. For example, there has been suggested a bright telephoto lens having an F number of 1.4 and the number of lenses in a focusing lens group is five; however, the telephoto lens is not light weight because there are too many lenses in the focus lens group. On the other hand, when the number of lenses in the focus lens group may be one or two in order to achieve the light weight, this provides a lens with an F number of about 2, which is not satisfactory in terms of brightness. Also, there is an example where the number of lenses in the focus lens group is two and the F number is 1.8; however, such a lens also needs a large aperture.
With respect to these demands, a design for a telephoto lens system applied to a single lens reflex (SLR) type camera has been suggested. For example, a back focus is designed to be long to accommodate a space for a mirror attracting light to an optical finder. Also, a design for a telephoto lens system that does not need a long back focus by using an electronic view finder instead of using the optical finder has been suggested recently.